This invention pertains to ballast circuits for gaseous electrical discharge devices, and more particularly, is concerned with inductive ballast circuits for direct current discharge devices.
Gaseous electrical discharges are found in a number of commercially important devices, particularly those used for artificial lighting. Most of the electrical discharge lamps in use today are powered by alternating current power mains via a ballasting device that supplies a controlled alternating current to the discharge. The ballasting device essentially places an inductive impedance in series with the lamp. This is necessary because the discharge lamps usually have a negative impedance characteristic which would lead to destructive current excursions if they are connected to a low impedance voltage source.
Operation of these lamps with an alternating discharge current necessitates some degree of compromise in electrode design because each electrode must alternately serve as anode and cathode. Some lamp designs being considered for commercial lighting application are to be operated with a unidirectional current in the discharge. The ballasting of such a direct current discharge lamp requires a circuit that gives the necessary current control without excessive power dissipation in the ballasting device. For this reason, the use of a series resistor as a ballast is clearly unsatisfactory as the voltage drop across the resistor and the associated power dissipation gives a low ballast-lamp system efficiency.
The act of merely substituting an inductor for a resistor raises other problems. For example, the circuit shown in FIG. 1 shows a dc discharge device 10 with an anode 11 and heated cathode 12. Power for heating the cathode is provided by a separate supply means 15. Inductor 16 is in series with the discharge and carries full wave rectified current from the diode bridge rectifier 17, the input of which is connected by terminals 18 and 19 to the ac power mains supplying, for example, 120 volts at 60 Hz. The discharge device contains a low pressure inert gas plus mercury vapor fill and if phosphor-coated on the inside of the envelope would be a fluorescent-type lamp. The electrical characteristic of such a low pressure discharge is such that it maintains an essentially constant voltage drop over a wide range of discharge currents, the voltage value depending on the discharge tube geometry and the composition of the gas fill. The operation of the simple ballast circuit of FIG. 1 depends on the relation between the input line voltage and the lamp voltage drop. In particular, if the average value of the voltage output from the full wave rectifier, that is its dc component, exceeds the device voltage drop, the discharge current will not be interrupted between successive half-cycles of the ac power line, the moreover, the dc component of lamp current resulting from the excessive dc component of the bridge output relative to the device voltage will be limited only by the dc resistance that may be present in the winding of the inductor 16. Another way to describe the difficulty with the circuit of FIG. 1 is to consider the buildup of energy stored in inductor 16. As the instantaneous output voltage from rectifier bridge 17 exceeds the device voltage drop, the lamp discharge begins to conduct current with a rate of increase in current proportional to the voltage across inductor 16. The current continues to increase until the rectifier output falls below the device voltage. Then the decreasing current induces an opposite polarity across the inductor which now adds to the rectifier output so as to equal the device voltage drop. Energy flows out of the inductor during the time interval near the zero crossing of the ac power line. If the energy has not been completely dissipated before the cycle repeats, there will be a net increase of stored energy and inductor discharge current per half-cycle of the ac line power and the cumulative effect will be an uncontrolled direct current increase resulting in possible component or device failure.